Deposited Polycrystalline Silicon Research Articles

Low-temperature polycrystalline silicon waveguides for low loss transmission in the near-to-mid-infrared region.

Low-temperature deposited polycrystalline silicon waveguides are emerging as a flexible platform that allows for dense optoelectronic integration. Here, the optical transmission properties of poly-silicon waveguides have been characterized from the near-to-mid-infrared wavelength regime, extending the optical transmission well beyond previous reports in the telecom band. The poly-Si waveguides with a dimension of 3 µm × ∼0.6 µm have been produced from pre-patterned amorphous silicon waveguides that are post-processed through laser melting, reflowing, and crystallization using a highly localized laser induced heat treatment at a wavelength of 532 nm. Low optical transmission losses (<3 dB cm-1) have been observed at 1.55 µm as well as across the wavelength range of 2-2.25 µm, aided by the relatively large waveguide heights that are enabled by the deposition process. The results demonstrate the suitability of low-temperature poly-silicon waveguides to find wide ranging applications within integrated mid-infrared systems.

Polycrystalline silicon PhC cavities for CMOS on-chip integration

In this work, we present an on-chip 2D and 3D photonics integration solution compatible with Front End of Line integration (FEOL) using deposited polycrystalline silicon (poly:Si) for optical interconnects applications. Deposited silicon integration on a bulk silicon wafer is here discussed in all its processing steps and configurations. Moreover, results of deposited silicon high-Q Photonic Crystal (PhC) resonators are shown, demonstrating the possibility to employ optical resonators patterned on this material in the next generation of 2D and 3D integrated optical interconnects.

Development of photodiode via the rapid melt growth (RMG) materials for energy conversion device

&lt;span&gt;Germanium (Ge) photodiodes were fabricated with the new RMG crucible materials that were established in this study. Results show that Ge large square patterns with size of 208 µm x 208 µm were unable to be achieved if ion implantation process was used in formation of photodiode. Delamination can be observed on all test samples during polycrystalline silicon (poly-Si) deposition at 620 &lt;sup&gt;o&lt;/sup&gt;C. This result was in contrast to a previous intrinsic Ge test structure, where good formation of squares with size similar to that 208 µm x 208 µm had been successfully attained even with high annealing temperature above 938 &lt;sup&gt;o&lt;/sup&gt;C. This indicates that doping through ion implantation has affected Ge film and caused delamination even at low temperature. However, good formations of Ge stripes were attained along with the ion implantation process in fabricating the photodiode. Results show that the sheet resistance of Ge stripe has significantly decreased compared to previous Ge resistors. The better resistance is due to the thicker (500 nm) Ge layer. In the case of Ge stripes with a p-i-n junction, only small fraction of test samples have shown a diode characteristic but with high leakage current. Results of I-V measurement show that a large fraction of test samples produced resistor characteristics. The high leakage current is believed due to the fact that the Ge films formed at this stage were polycrystalline in structure. Thus the grain size of Ge stripe has affected the performance. Slight changes on the I-V characteristic of single Ge can be observed when samples were tested under illumination. Therefore, it has potential to be used for future development of energy conversion device. &lt;/span&gt;

Thermodynamic Simulation of Polycrystalline Silicon Chemical Vapor Deposition in Si–Cl–H System

Based on thermodynamic data for related pure substances, the relations of (nCl/nH)Eq and (nCl/nH)o have been plotted in the Si–Cl–H system. The results show that the difference of (nSi/nCl)o and (nSi/nCl)Eq is the driving force for polycrystalline silicon chemical vapor deposition (CVD). SiHCl3 is preferred for polycrystalline silicon deposition to SiCl4. SiH2Cl2 would be even better, but it is not stable as a gas and hence it is less frequently used. Then, thermodynamic simulation of polycrystalline silicon CVD in the Si–H–Cl system has been investigated. The pressure has a negative effect on polycrystalline silicon yield. The optimum temperature is 1400 K, at which the kinetic rate of rate-determining step for the main reaction is large enough. The excess hydrogen is necessary for polycrystalline silicon CVD in the Si–Cl–H system. However, the silicon deposition rate increases then decreases with increasing H2 molar fraction. The optimum H2 molar fraction should be determined by considering thermodynamics and transport phenomena simultaneously. Finally, the optimum conditions have been obtained as 1400 K, about 0.1 MPa, and H2 to SiHCl3 ratio of 15, which are close to the limited reported values in the open literature. Under the optimum conditions, the silicon yield ratio is 34.82% against 20% reported in the open literature.

Influence of Slim Rod Material Properties to the Siemens Feed Rod and the Float Zone Process

The identification and understanding of material properties influencing the float zone process is important to crystallize high purity silicon for high efficiency solar cells. Also the knowledge of minimal requirements to crystallize monocrystalline silicon with the float zone process is of interest from an economic point of view. In the present study, feed rods for the float zone process composed of a central slim rod and the deposited silicon from the Siemens process are investigated. Previous studies have shown that the slim rod has a significant impact on the purity and suitability for further crystallization processes. In particular, contaminations like substitutional carbon and the presence of precipitates as well as the formation of oxide layers play an important role and are investigated in detail. For this purpose different slim rod materials were used in deposition and float zone crystallization experiments. Samples were prepared by cross sectioning and core drilling of Siemens rods, which were recrystallized with the float zone process. Recrystallized drilled cores are analyzed with FT-IR spectrometry concerning the carbon and oxygen content. To estimate the grain growth behavior on the slim rod surface in dependence of the used slim rod material, EBSD mappings inside a SEM are performed on squared and circular slim rods. TEM analysis was used to investigate the presence of an oxide layer at the interface between slim rod and deposited polycrystalline silicon. Additionally the influence of a nitrogen-containing gas atmosphere during the slim rod pulling is investigated by IR microscopy and ToF-SIMS regarding Si3N4 precipitation.

Characterization of thin polycrystalline silicon films deposited on glass by CVD

We deposited polycrystalline silicon (poly-Si) thin films on commercial float glass by chemical vapour deposition from trichlorosilane at temperatures between 735 and 870 °C. The structural properties of the films were evaluated by means of scanning electron microscopy, x-ray diffraction, atomic force microscopy, reflectance in the ultraviolet region and Raman spectroscopy. The electrical characterization involved measurements of dark conductivity and photoconductivity as a function of temperature, Hall effect, ambipolar diffusion length from the steady-state photocarrier grating technique and density of defects by means of modulated photoconductivity. By using boron tribromide as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained. The process, the reactants and the substrate used are of low cost, and proved to be adequate for direct poly-Si deposition, giving films of good structural and electrical properties.

Formation of high purity Si nanofiber from metallurgical grade Si by molten salt electrorefining

We studied the purification of metallurgical grade Si from solutions of K2SiF6 in fluoride melts using a molten salt electrorefining process at 700 °C. Electrorefining close to the deposition potential gave dense, coherent, and well-adherent deposits. It was shown that the deposition rate and microstructure of Si strongly depend on the process temperature. Deposited polycrystalline silicon has a well defined rod shape and crooked structure that varies with current density. The anodic dissolution rate is affected by the initial concentrations of K2SiF6 and the applied current density. The results of an inductively coupled plasma (ICP) analysis indicated that recovered silicon fiber deposits with purities greater than 99.98% can be obtained using the developed technique. The morphology of the electrodeposited silicon on silver substrates is discussed in the context of a cathodic reaction on the electrode surface, and a comprehensive explanation of the purification mechanism with salt removal is provided.

Polycrystalline silicon thin films on glass deposited from chlorosilane at intermediate temperatures

We show that commercial float glass can be used as a substrate to deposit polycrystalline silicon (poly-Si) by chemical vapor deposition from trichlorosilane at temperatures between 740 and 870 °C. By using scanning electron microscopy an average grain size lower than 0.4 μm was observed, with a columnar structure suitable for the electrical conduction in photovoltaic cells. X-ray diffraction reveals a strong (2 2 0) preferential orientation of the films, which is indicative of a low density of intra-grain defects. Atomic force microscope images reveal a conical structure, with a root mean square roughness of ~65 nm for samples of around 3 μm in thickness. This natural texture is a positive characteristic from the point of view of light trapping. By using boron tribromide as a doping agent, degrees of doping ranging from intrinsic to clearly p-doped were obtained, as shown by dark conductivity measurements as a function of temperature. The process, the reactants and the substrate used are of low cost and proved to be adequate for direct poly-Si deposition.

Polycrystalline silicon films on glass grown by amorphous–liquid–crystalline transition at temperatures below 330 °C

An approach to deposit polycrystalline silicon layers on amorphous substrates is presented. It is shown that metastable amorphous silicon can be transformed into its more stable microcrystalline structure at a temperature below 330 °C via an intermediate liquid solution stage. In particular, the interaction of liquid indium nanodroplets in contact with amorphous silicon is shown to lead to the formation of circular polycrystalline domains. Crystallinity of these domains is analyzed by micro-Raman spectroscopy. The droplet size necessary for the onset of crystallization is related to the temperature of the film. Full coverage of the substrate with microcrystalline silicon has been obtained at 320 °C within less than one hour. These films might act as seeding layers for further enlargement by steady-state liquid phase epitaxy.

Effect of Boundary Layers on Polycrystalline Silicon Chemical Vapor Deposition in a Trichlorosilane and Hydrogen System

This paper presents the numerical investigation of the effects of momentum, thermal and species boundary layers on the characteristics of polycrystalline silicon deposition by comparing the deposition rates in three chemical vapor deposition (CVD) reactors. A two-dimensional model for the gas flow, heat transfer, and mass transfer was coupled to the gas-phase reaction and surface reaction mechanism for the deposition of polycrystalline silicon from trichlorosilane (TCS)-hydrogen system. The model was verified by comparing the simulated growth rate with the experimental and numerical data in the open literature. Computed results in the reactors indicate that the deposition characteristics are closely related to the momentum, thermal and mass boundary layer thickness. To yield higher deposition rate, there should be higher concentration of TCS gas on the substrate, and there should also be thinner boundary layer of HCl gas so that HCl gas could be pushed away from the surface of the substrate immediately.

Waveguide-integrated telecom-wavelength photodiode in deposited silicon

We demonstrate photodiodes in deposited polycrystalline silicon at 1550 nm wavelength with 0.15 A/W responsivity, 40 nA dark current, and gigahertz time response. Subband absorption is mediated by defects that are naturally present in the polycrystalline material structure. The material exhibits a moderate absorption coefficient of 6 dB/cm, which allows the same microring resonator device to act as both a demultiplexing filter and a photodetector. We discuss the use of deposited silicon-based complementary metal-oxide semiconductor materials for nanophotonic interconnects.

Dispersion of nanospheres on large glass substrate by amorphous or polycrystalline silicon deposition for localized surface plasmon resonance

Nanosphere lithography is a cost-effective way to fabricate noble metal nanostructures for plasmonics. However, dispersing nanospheres on a large area of glass substrate is a difficulty encountered when transparent substrate is required in applications such as localized surface plasmon resonance or surface enhanced Raman spectroscopy. Because poly(diallyldimethyl ammonium chloride) (PDDA) on silicon surface introduces a force that can disperse nanospheres on silicon, in this article, we modify the glass surface through amorphous or polycrystalline silicon deposition and thus well disperse polystyrene or silica nanospheres over a glass area of more than 2 cm × 2 cm. Transmission loss of the glass substrate caused by amorphous or polycrystalline silicon deposition is analyzed with good agreement to experimental spectra, and localized surface plasmon resonance signals generated from the gold nanostructures fabricated on these substrates are measured and yield a sensitivity of 317 nm/RIU, which prove the feasibility and effectiveness of our method.

Fabrication of poly-Si films by continuous local thermal chemical vapor deposition on flexible quartz glass substrate

The continuous deposition of polycrystalline silicon film on quartz fiber by local thermal chemical deposition was investigated. High-speed deposition owing to high temperature and locality was examined using fixed and moving substrates. We confirmed the high-speed deposition of polycrystalline silicon and achieved a maximum speed of over 1μm∕s. Furthermore, we succeeded in a continuous deposition of polycrystalline thin silicon with a thickness of 50–100nm on a quartz fiber with low roughness and low impurity content. Thin film transistor with a mobility more than 3.7cm2∕Vs was achieved by using this film.

Germanium Bonding to AL2O3

Germanium has been bonded to both single crystal Al2O3 (sapphire) as well as fine grain Al2O3. A germanium to sapphire bonding energy of 3 J/m2 has been measured after a 200 oC bond anneal. Micro voids formed between the germanium/sapphire interface can be removed by employing an interfacial layer of silicon dioxide on either surface. Patterning the sapphire into a grid pattern prior to bonding creates an escape path for trapped gas or moisture allowing micro void free direct bonding to be achieved. Modifying the surface of the fine grain Al2O3 surface with a polycrystalline silicon deposition and polish creates a surface, having an rms roughness of 1.5nm(measured over a 250µm square area), suitable for bonding. Techniques employed in the germanium sapphire bonding can then be used in the bonding of fine grain Al2O3 to germanium.

Excimer laser annealing of microcrystalline silicon

AbstractThe development of flexible displays and screens is now a major target of the industry. This purpose requires the elaboration of pliant substrate compatible processes. The main obstacle is the vulnerability of such substrates toward heat. Most parts of usual processes are able to be performed at low temperature, but polycrystalline silicon deposition always involves high temperature. Polycrystalline silicon is necessary to reach high mobility, which, in the case of flexible screens, is critical to decrease the area of the pixel circuit and then to reach high aperture ratio. One solution consists in low temperature deposition of microcrystalline silicon followed by an excimer laser crystallization phase. Microcrystalline silicon films were deposited at low temperature (165 °C maximum) by radio‐frequency plasma enhanced chemical vapour deposition on glass substrate covered with silicon dioxide film and on polyethylene naphtalate covered with silicon dioxide film as flexible substrate. These microcrystalline silicon films were excimer laser crystallized, using a step‐by‐step method. Polyethylene naphtalate substrates underwent crystallization without showing any damage. Using deposits made on glass, the crystallization technique has been optimised to obtain the highest crystalline quality starting from 200 nm thick microcrystalline silicon films. On each kind of substrate, crystallized films were characterized using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. Raman spectrum shows very sharp transverse optic peak at 521 cm–1. The small full width at half maximum, 4 cm–1, is close to the value for single crystalline silicon. Scanning electron microscopy and atomic force microscopy reveal silicon grains with size over 400 nm on glass and 100 nm on polyethylene naphtalate. Thin film transistors with a mobility of 400 cm²/V.s on glass and 46 cm²/V.s on polyethylene naphtalate have been produced from such crystallized layers. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-speed all-optical modulation using polycrystalline silicon microring resonators

We experimentally demonstrate on-chip active photonic devices fabricated from deposited polycrystalline silicon, which can be used for monolithic three-dimensional integration of optical networks. The demonstrated modulator is based on all-optical carrier injection in a micrometer-size resonator and has a modulation depth of 10dB and a temporal response of 135ps. Grain boundaries in the polycrystalline silicon (polysilicon) material result in faster electron-hole recombination, enabling a shortened carrier lifetime and a faster optical switching time compared to similar devices based on crystalline silicon.

Rapid Thermal Annealing and Hydrogen Passivation of Polycrystalline Silicon Thin-Film Solar Cells on Low-Temperature Glass

The changes in open-circuit voltage (Voc), short-circuit current density (Jsc), and internal quantum efficiency (IQE) of aLuminum induced crystallization, ion-assisted deposition (ALICIA) polycrystalline silicon thin-film solar cells on low-temperature glass substrates due to rapid thermal anneal (RTA) treatment and subsequent remote microwave hydrogen plasma passivation (hydrogenation) are examined. Voc improvements from 130 mV to 430 mV, Jsc improvements from 1.2 mA/cm2 to 11.3 mA/cm2, and peak IQE improvements from 16% to &gt; 70% are achieved. A 1-second RTA plateau at 1000°C followed by hydrogenation increases the Jsc by a factor of 5.5. Secondary ion mass spectroscopy measurements are used to determine the concentration profiles of dopants, impurities, and hydrogen. Computer modeling based on simulations of the measured IQE data reveals that the minority carrier lifetime in the absorber region increases by 3 orders of magnitude to about 1 nanosecond (corresponding to a diffusion length of at least 1 μm) due to RTA and subsequent hydrogenation. The evaluation of the changes in the quantum efficiency and Voc due to RTA and hydrogenation with computer modeling significantly improves the understanding of the limiting factors to cell performance.

The influence of boron concentrations on structural properties in disorder silicon films

In this work we present a detailed structural of a series of B-doped hydrogenated microcrystalline silicon (μc-Si:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and B-doped polycrystalline silicon (poly-Si) films produced by step-by-step laser crystallization process from amorphous silicon. The influence of doping on the structural properties and structural changes during the sequential crystallization processes were monitored by Raman spectroscopy. Unlike μc-Si:H films, that consist of a two-phase mixture of amorphous and ordered Si, partially crystallized sample shows a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. With increasing doping concentration the LO-TO phonon line in poly-Si shift to smaller wave numbers and broadens asymmetrically. The results are discussed in terms of resonant interaction between optical phonons and direct intraband transitions known as a Fano resonance. In μc-Si:H films, on the other hand, the Fano effect is not observed. The increase of doping in μc-Si:H films suppressed the crystalline volume fraction, which leads to an amorphization in the film structure. The structural variation in both μc-Si:H and poly-Si films leads to a change in hydrogen bonding configuration.

Incubation Effects upon Polycrystalline Silicon on Glass Deposited by Hot‐Wire CVD

AbstractA growth mechanism diagnosis for high‐rate, polycrystalline silicon deposition using hot‐wire CVD is explored in this article. The effects of various deposition parameters on the Si film growth are investigated by Raman spectroscopy and transmission electron microscopy (TEM) measurements, with special attention paid to the crystalline and amorphous phases. It is found that the H2/SiH4 ratio and substrate temperature (Ts) are the essential process parameters in determining the crystalline phase and the incubation thickness of the as‐deposited Si film. Under low hydrogen dilution conditions, an amorphous incubation layer was formed and degraded the crystalline fraction of the Si film. The incubation thickness prior to the nucleation decreases with the increase of the hydrogen dilution ratio. Under higher H2/SiH4 ratios (≥50), the amorphous phase deposited on the substrate could be etched selectively by the atomic hydrogen. As a consequence, nucleation of small crystallite occurs directly on the substrate. At low substrate temperatures, the deposition process shows the disadvantages of small grain size and low crystallinity. The poly‐Si film can be deposited from the seed layer under high Ts conditions (&gt;350 °C) where the Si appears singly crystallized at the initial stage and then grows epitaxially.

Time-resolved reflectance and transmittance measurements of laser-induced free carriers in germanium, silicon, and zinc selenide at 106 μm

We present experimental results of reflectance and transmittance measurements of infrared radiation by high-density photogenerated free carriers in polycrystalline germanium, polycrystalline silicon, and chemical vapor deposition zinc selenide windows. Linearly polarized 1064 and 532 nm wavelength light from a Nd:YAG laser with a 130 ps pulse width were used to generate free carriers in the samples. Reflectance and transmittance were measured at a 10.6 microm wavelength using a linearly polarized CO2 laser.